Apparently Brad was being modest with his response. He actually has a much more detailed blog post on this very topic. It is VERY detailed and was a huge help to me (thanks Brad!). You can find his post here:

Hi Brad, I have tried this as well and I cannot seem to see the changes on the site. Is shows it in the custom header area “Images of exactly 920 × 150 pixels will be used as-is.” and I made the change in the style.css as well and it still doesnt show up on the site. Any advice, thanks in advance. Tony